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mechanism

Wood

A nail driven into a trunk stays at the same height for the life of the tree. Trunks do not grow upwards; they grow outwards, in a layer a few cells thick.

Wood is plumbing that has been left in place. It is dead xylem — pipes whose cells died on completion — stacked up year after year, and a trunk grows in a layer a few cells thick just under the bark while everything inside it stays finished.

Almost everything surprising about a trunk follows from one fact: the tree is growing in a thin cylinder near the surface, and the enormous structure inside that cylinder is not growing at all. A nail hammered into a trunk stays at the same height for the rest of the tree’s life. The wood at the centre stopped conducting water decades or centuries ago and has been loaded with compounds that make it hard to rot. The cells that carry water upwards are dead by the time they start working, which is not a defect but the design — an empty tube conducts better than a full one. Read outward from the centre and you are reading a history: heartwood the tree has retired, sapwood it is still using, one thin living layer making more, then the phloem carrying sugars down and the bark keeping the whole thing from drying out. Almost every practical question about wood — why some is dense, why oak lasts and pine does not, why a hollow tree is still alive — is answered somewhere in that sequence.

Developed record · 76% complete · reviewed 2026-08-30

What this page covers

Wood is a tissue, not a group. It is secondary xylem, produced by a cambium, and it has evolved in conifers and flowering plants independently enough that "wood" covers structurally different materials. Palms, bamboos and tree ferns have no true wood.

Often confused with: Palm stems; Bamboo culms; Tree fern trunks

Quick facts

What it is
Secondary xylem — water-conducting tissue, dead at maturity
Where growth happens
The cambium, a layer a few cells thick under the bark
Dry mass
Roughly half carbon, taken from the air
Why density varies
Partly the pressure its pipes must resist without collapsing

What you are looking at when you look at a trunk

Five layers, and only one of them is growing.

A trunk is built outward from the middle, so a cross-section reads as a sequence in time. At the centre is heartwood: xylem the tree has taken out of service, plugged and loaded with resins and tannins that resist decay. Around it is sapwood, the part still conducting water. Between sapwood and bark is the cambium, a cylinder of dividing cells a few cells thick — the only part of the trunk that is alive in the sense of growing. It makes wood inward and phloem outward. The phloem carries sugars down from the leaves, and the bark, made from a second cambium of its own, keeps everything inside from drying out.

A trunk grows in one thin layer just under the bark. Everything inside it is finished.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Secondary thickening in woody plants occurs at the vascular cambium, a lateral meristem a few cells thick, which produces xylem inward and phloem outward; the wood interior to it consists of cells that have completed differentiation and, in the case of mature xylem, are dead at maturity.

Who this applies to
Trees with true secondary growth. Palms, tree ferns and bamboos thicken differently and are excluded.
Studied in
Pinophyta, Magnoliophyta

You may have heard

“A tree grows all over.”

A nail driven into a trunk stays at the same height for the life of the tree. Trunks thicken from a layer under the bark and lengthen only at the shoot tips, so the rest of the wood is a finished structure the tree is standing on.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Basic, long-settled plant anatomy, visible in any section of a trunk and consistent across the literature.

How far it can be extended

Secondary thickening from a vascular cambium is the shared mechanism across conifers and woody flowering plants.

Caveats

  • Living parenchyma cells persist within sapwood and do real work in storage and defence, so "dead inside" is a simplification of the xylem specifically.
  • Palms have no vascular cambium and cannot heal a wound the way an oak can.

Last reviewed 2026-08-30

The evidence (1 study)

  • Supports · primary

    Xylem Structure and the Ascent of Sap

    Tyree and Zimmermann, 2002 · Springer

    Sets out the structure of the stem, the position of the cambium and the state of mature xylem, in the course of explaining how water moves through it.

Diagram

A trunk in cross-section

Read from the outside in: bark, phloem, cambium, sapwood, heartwood. Not to scale — the cambium is far thinner than any drawing can show.

BarkPhloemcarries sugars down; living tissueCambiuma layer a few cells thick — the onlypart of a trunk that is growingSapwoodconducting xylem: the cells are dead,the tissue is workingHeartwoodno longer conducts; loaded withcompounds that resist decayGrowth ringswide early wood, dense late wood —one pair per seasonin a seasonal climate only
The same explanation in words

A circular cross-section through a trunk, labelled in five concentric layers. The outermost is bark. Just inside it is the phloem, which carries sugars downward and is living tissue. Between phloem and wood is the cambium, marked as a single thin line and highlighted, because it is only a few cells thick and is the sole part of the trunk that is growing. Inside the cambium is sapwood — conducting xylem, whose cells are dead but whose tissue is working — with several growth rings drawn as concentric lines within it, each ring a pair of wide early wood and dense late wood. At the centre is heartwood, shaded darker: no longer conducting, filled with compounds that resist decay. A note records that growth rings are annual only in a seasonal climate; a tree in an aseasonal tropical climate may have no usable rings at all.

The cells that carry water up a tree are dead before they carry anything. A xylem conduit finishes developing by dissolving its own contents, leaving an empty tube — which is exactly what you want a pipe to be.

Based on A trunk grows in one thin layer just under the bark. Everything inside it is finished.
Words used here
Cambium
A cylinder of dividing cells just under the bark, a few cells thick. It makes wood inward and phloem outward, and it is the only growing part of a trunk.
Secondary xylem
The technical name for wood: water-conducting tissue laid down year after year by the cambium.
Phloem
The living tissue just inside the bark that carries sugars from the leaves to the rest of the tree.

Why a hollow tree is not a dying tree

The centre of a big trunk is dead, and the tree is fine — a hollow tree can be perfectly healthy.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Heartwood is xylem that has ceased conducting and has been infiltrated with extractive compounds; it provides mechanical support and resists decay but takes no part in transport, so its loss to decay does not directly impair the physiology of the tree.

Who this applies to
Heartwood-forming trees. Not all species form a distinct heartwood.
Studied in
Pinophyta, Magnoliophyta

You may have heard

“A hollow tree is a dying tree.”

The wood that rots out of the middle was already dead and was not moving water. Hollow veteran trees are often centuries old, still growing and among the richest habitats a woodland has.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Long-settled wood anatomy, and directly observable in the very large number of ancient hollow trees that are alive and reproducing.

How far it can be extended

Heartwood formation is widespread across woody plants, though the extractives involved differ by species.

Caveats

  • A hollow trunk is physiologically fine but mechanically weaker, which matters for a tree near a path or a building.
  • Decay that reaches the sapwood is a different matter, because that tissue is conducting.

Still unanswered

  • How much the compounds that make heartwood decay-resistant vary in effectiveness between species and sites.

Last reviewed 2026-08-30

The evidence (2 studies)

  • Supports · primary

    Xylem Structure and the Ascent of Sap

    Tyree and Zimmermann, 2002 · Springer

    Describes the transition from conducting sapwood to non-conducting heartwood and the consequences for transport.

  • Context · supporting

    Ecology of coarse woody debris in temperate ecosystems

    Harmon et al., 1986 · Advances in Ecological Research

    Documents the decay of wood in living and dead trees and the specialist communities that depend on the resulting cavities.

Heartwood is where a tree puts the wood it has finished with. As sapwood ages it stops conducting; the tree fills those conduits and loads the tissue with compounds — tannins, resins, phenolics — that make it unpalatable to fungi. That is why oak heartwood outlasts oak sapwood by a very long way, and why the durability of a timber is a statement about a tree’s chemistry rather than about its density.

It also explains the ancient hollow trees that look, to most people, like a tree in trouble. Fungi eventually get past the defences and take the heartwood out. What they remove was already dead and was moving nothing, so the tree carries on: the conducting sapwood and the cambium are near the surface and untouched. A hollow veteran oak can be several centuries old, still growing, still producing acorns, and holding a community of insects, bats and birds that no young tree can support.

What is doing the hollowing out

  • Decomposition

    White rot, brown rot, and what each one leaves behind

  • Fungi

    The organisms that can digest wood, and why almost nothing else can

Words used here
Heartwood
The inner wood a tree has taken out of service, plugged and chemically loaded to resist decay. It supports the tree and conducts nothing.
Sapwood
The outer wood that is still conducting water. Its cells are dead; the tissue is working.

Why some wood is heavy and some floats

Density is partly a decision about how much tension the plumbing must survive.

Part of the reason some wood is so heavy is that its pipes have to resist being crushed.

Well supported

Good evidence backs this, though some details remain open.

Across conifers and flowering plants, the thickness of conduit walls relative to conduit diameter — and therefore wood density — scales with the negative pressure a species routinely sustains, consistent with a mechanical requirement to resist implosion of water-conducting cells.

Who this applies to
Woody species across conifers and flowering plants, sampled mainly in temperate and Mediterranean floras.
Studied in
Pinophyta, Magnoliophyta
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

A strong and mechanically motivated correlation across species, but the causal step is inferred from engineering rather than demonstrated by manipulation, and wood density serves several functions at once.

How far it can be extended

The relationship holds across two lineages that build their conduits in structurally different ways, which is what makes a mechanical rather than a phylogenetic explanation likely.

Caveats

  • Wood density also reflects mechanical support, storage and defence, which this does not separate.
  • Sampling is weighted towards temperate species.

Still unanswered

  • How much of the variation in tropical hardwood density is hydraulic and how much is defensive.

Last reviewed 2026-08-30

The evidence (1 study)

The water inside a conducting cell is under tension — stretched, at a pressure below zero — while the air outside pushes inward at ordinary atmospheric pressure. A conduit must be built not to collapse under that difference, and the more negative the pressure a species routinely runs at, the thicker its conduit walls have to be relative to their width. Thicker walls mean denser wood.

  • Wide conduits move a great deal of water and fail easily; narrow ones are slow and safe. Ring-porous trees such as oak gamble on very wide early-season vessels and lose them readily.
  • Conifers use narrow tracheids rather than vessels, which is slower and much more tolerant of freezing — one reason conifers dominate cold forests.
  • Density also carries mechanical support and chemical defence, so heavy wood is never explained by hydraulics alone.
  • Balsa is at one extreme and lignum vitae at the other, and both are flowering plants: this is a difference in strategy, not in ancestry.

The tension itself

  • Trees

    How water is pulled to the top, and why there is a ceiling

Words used here
Tracheid
A single-celled water conduit, used by conifers. Narrower and shorter than a vessel, and much less prone to failing when sap freezes.
Vessel
A water conduit built from many cells stacked end to end with the walls between them dissolved. Found in flowering plants; wide, fast and vulnerable.

What a growth ring records, and what it does not

A growth ring is a pair of tissues rather than a line. In spring a temperate tree lays down wide, thin-walled conduits to move water while it is plentiful; later in the season it makes narrower, thicker-walled, denser wood. The abrupt change from late wood to the next spring’s early wood is the boundary the eye reads as a ring.

Because ring width responds to conditions, a sequence of rings is a record of the years a tree lived through, and matching those sequences between trees — crossdating — is how tree-ring chronologies are built and how timber in a building can be dated to a season. The limits matter as much as the method. A tree in an aseasonal tropical climate may lay down no usable rings at all. A tree that had a very bad year may lay down none, or may produce a false ring within one season. Counting rings gives a careful minimum age, not a birthday.

Where ring counting runs into its limits

  • Trees

    Why the oldest-tree question depends on what is being counted

Words used here
Crossdating
Matching the pattern of wide and narrow rings between trees to fix each ring to a calendar year, rather than counting outward from a guess.
Dendrochronology
Dating by tree rings. It provides the calendar against which much of archaeology and past climate is measured.

Cellulose, lignin, and why wood outlasts everything else a plant builds

Wood is mostly two substances. Cellulose is a long chain of sugar units, laid down as fibres, and it provides tensile strength — the resistance to being pulled apart. Lignin is a dense, irregular polymer packed around and between those fibres, and it provides stiffness, water resistance and, most consequentially, a chemistry that almost nothing can digest. A leaf rots in a season. A trunk of the same tree can lie on a forest floor for decades, and the difference is lignin.

A tree is built mostly out of air — the carbon in wood came from carbon dioxide

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

The dry mass of a plant is predominantly carbon, hydrogen and oxygen. Carbon enters exclusively as atmospheric carbon dioxide fixed in photosynthesis; hydrogen and oxygen derive from water. Soil supplies mineral nutrients required in comparatively small quantities, not the bulk of the material.

Who this applies to
plants generally, trees most strikingly
Studied in
Plantae

You may have heard

“Trees get their mass from the soil”

Van Helmont tested this in the 1640s by growing a willow in weighed soil for five years. The tree gained 74 kg; the soil lost about 60 g. He concluded the mass came from water, which was half right — most of the dry mass is carbon, and it came out of the air.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Established by the carbon-fixation work and by elemental analysis of biomass. Van Helmont’s seventeenth-century willow experiment showed the soil loses almost no mass, though he drew the wrong conclusion from it.

How far it can be extended

Elemental composition of plant biomass is general, and the carbon source is established by isotope tracing.

Caveats

  • Soil nutrients are essential; the claim concerns bulk mass, not necessity.
  • A large fraction of a living tree’s mass is water, which does come through the roots.
  • Mineral nutrient limitation constrains growth even though it contributes little mass.

Still unanswered

  • How will rising atmospheric carbon dioxide alter growth where nutrients limit it?

Last reviewed 2026-08-10

The evidence (2 studies)

Both substances are built from sugars made in the leaves, which is why the carbon in a plank was in the atmosphere before it was in a tree. That is also why decomposition is the second half of the story: everything a tree assembles has to be taken apart again, and for several hundred million years there were organisms making lignin and comparatively few capable of dismantling it.

Both halves of the cycle

  • Trees and carbon

    Where the carbon in wood came from, and where it goes next

  • Decomposition

    The organisms that can take lignin apart, and what they leave

Words used here
Cellulose
Chains of glucose laid down as fibres. The most abundant organic polymer on Earth, and the tensile element of wood.
Lignin
A dense, irregular polymer packed around cellulose fibres. It makes wood stiff and waterproof, and it is very difficult to digest.

What is still unsettled about wood

  • How much of the variation in tropical hardwood density is hydraulic?

    Why it matters: Wood density predicts how much carbon a forest holds, and it is used in every biomass equation. If density is set as much by defence as by plumbing, the equations are fitting something they do not represent.

    What would settle it: Measurements of conduit wall thickness, extractive content and drought exposure in the same individuals across a tropical species range.

  • What decides when sapwood becomes heartwood?

    Why it matters: The transition sets how much conducting tissue a tree keeps and how decay-resistant its timber becomes, and the trigger is not well characterised.

  • Can the mechanical and hydraulic roles of dense wood be separated experimentally?

    Why it matters: They are correlated across species and the causal split is inferred from engineering rather than tested.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

10 studies, newest first. Each one has a page explaining what it found and what it could not show.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this record is, and what it is still missing

NatureHQ publishes its own gaps. This record is at 76% completeness against what we would call a finished subject, and was last reviewed on 2026-08-30. It carries 8 claims and answers 5 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • Timber use, seasoning and workability are heavily searched and deliberately out of scope; this page is about the living tissue.
  • Bark is covered only as the outermost layer, and its own structure and ecology would support a section.
  • Reaction wood — the tissue a leaning tree builds to right itself — is a good story and is absent.